This invention relates to a method of restarting an induction motor in the decelerating mode by a variable frequency electric source and also to an apparatus for carrying out such a method.
There are widely utilized variable speed control systems for operating an induction motor by an associated variable frequency electric source such as an inverter, a cyclo-converter or the like. In those systems, the induction motor in the decelerating mode is required to be again supplied by the variable frequency electric source (which is hereinafter typical of an inverter) in the following cases:
(a) If a failure of commercial electric power has instantaneously occurred, the inverter is suspended at a time and then, upon the recovery of power supply, the induction motor is required to return to its original mode of operation before its stop.
(b) When it is desired to change the induction motor operated by a commercial electric source with a frequency of 50 or 60 Hz by this time to be operated at another frequency, for example, of 30 Hz, the inverter picks up the induction motor which has disconnected from the commercial electric source to coast and decelerate.
When the inverter supplies electric power to the induction motor operated in the decelerating mode, the following problems have arisen:
(1) Since a speed of the induction motor must be precisely detected in order to connect the motor to the inverter, it is necessary to use a tachometer generator for detecting the number of rotations per unit time of the induction motor. The use of the tachometer generator, however, is uneconomical and also undesirable because tachometer generators are generally less reliable than induction motor.
(2) Even though it is assumed that a tachometer generator or a pulse generator for generating pulses proportional to the number of rotations per unit time of the induction motor is used to detect a speed of the induction motor, a large-scale induction motor has a high restarting current flowing therethrough due to a low slip frequency unless the operating frequency of the inverter is precisely adjusted to the speed of the induction motor in the decelerating mode. On the other hand, upon restarting small-sized, high resistance induction motors after an instantaneous failure of electric power, one has previously employed a method of restarting the inverter at its original frequency to effect the soft-start of a voltage alone to thereby increase the number of rotations per unit time of the induction motor to its original magnitude. In large-scale low resistance induction motors, however, it is required to adjust precisely the restarting frequency of the inverter to the speed of the induction motor.
(3) Even if an attempt is made to render the restarting frequency of the inverter equal to the number of rotations per unit time of the induction motor as precisely as possible, as described above in the preceding paragraph (2), usual analog methods have errors ranging from .+-.0.1 to .+-.0.3%. In case of a negative slip error, the restarting of the inverter has resulted in the regenerative mode of operation. This has led to the necessity of forming a DC source for the inverter of a double converter which is enabled in the regenerative mode. If the DC source does not include regenerative means, then the resulting regenerative power has caused the DC overvoltage state. Therefore, it has been generally known that the restarting of induction motors should occur with a positive slip maintained.
Accordingly it is an object of the present invention to eliminate the disadvantages of the prior art practice as described above by the provision of a new and improved method of restarting an electric induction motor operated in the coasting and decelerating mode by a variable frequency electric source in such a manner that the variable frequency electric source is prevented from becoming inoperative due to an overcurrent on an overvoltage.
It is another object of the present invention to provide an apparatus for carrying out the method as described in the preceding paragraph.